This invention relates to oxide coated fine metal particles which comprise fine core metal particles coated with an oxide, a complex oxide or an oxy-acid salt of a dissimilar metal or a complex oxide or a complex salt of oxides of the core metal and a dissimilar metal. The invention also relates to a process for producing such oxide coated fine metal particles.
Heretofore, coated metal particles comprising core particles made of inorganic materials such as diamond and ceramics or metals and which are coated with various metallic materials or inorganic materials such as ceramics, oxides, carbides and nitrides that serve as sintering aids or thermal spraying aids have been used in diverse fields including the manufacture of sinters such as electrical insulating materials (e.g., semiconductor substrates, printed wiring circuit boards and various other electrically insulated components), machining materials of high hardness and precision (e.g., cutting tools, dies and bearings), functional devices (e.g. grain boundary capacitors and humidity sensors) and precision sintered moldings, as well as the manufacture of thermal sprayed parts such as engine valves that require wear resistance at elevated temperatures. The use of such coated particles contributes to increase not only the strength of bond between dissimilar ceramics or metals in sinters and thermal sprayed parts but also their denseness.
Unexamined Published Japanese Patent Application (kokai) No. 253851/1996 discloses a composite powder for thermal spray having an average particle size of 10-150 xcexcm. that comprises Ti particles having a Ni coating layer of 5 xcexcm or more with the ratio between the size of Ti particles and the thickness of Ni layer being no more than 10. Unexamined Published Japanese Patent Application (kokai) No. 253853/1996 discloses a composite powder for thermal spray comprising Coxe2x80x94Cr based alloy particles with an average size of 20-99 xcexcm that are coated with partly embedded WC particles having an average size of 0.5-20 xcexcm. To produce these composite powders for thermal spray, the powders of the two starting materials are confined in a stirring vessel either directly or after being mixed uniformly with a mixer and thereafter agitated with a stirrer so that the coating particles are mechanically urged and compressed against the core particles, thereby achieving mechanical coating of the latter.
Commonly assigned Unexamined Japanese Patent Application (kokai) Nos. 75302/1991, 53268/1995-54008/1995, etc. disclose coated particles comprising the particles of an inorganic or metallic material with an average size of 0.1-100 xcexcm that are coated with the superfine particles of a similar or dissimilar inorganic or metallic material having an average size of 0.005-0.5 xcexcm, as well as processes for producing such coated particles. The processes disclosed in these patents comprise the steps of generating the superfine particles by a vapor-phase method such as a thermal plasma method, introducing the core particles into the stream of the generated superfine particles, and contacting the two kinds of particles in a fluid state so that the surfaces of the core particles are coated with the superfine particles.
The composite powders for thermal spray that are disclosed in Unexamined Published Japanese Patent Application Nos. 253851/1996 and 253853/1996, supra, are no more than those produced by mechanically urging and compressing coating particles such as Ni or WC particles against core particles such as Ti or Coxe2x80x94Cr based alloy particles until a mechanical coating is produced. The adhesion between the core and coating particles at their interface is weak and, as a further problem, the size of the core particles is as large as several micrometers to a hundred-odd micrometers and the coating particles are also limited to those which are no smaller than 0.5-20 micrometers. In addition, the core particles are metal and the disclosure about the coating particles is limited to metals and carbides thereof; in other words, the surfaces of core metal particles are not coated with an oxide of a dissimilar metal.
Speaking of the coated particles disclosed in commonly assigned Japanese Patent Application Nos. 75302/1991, 53268/1995-54008/1995, etc., supra, the coating particles are as fine as 0.005-0.5 xcexcm in average size since they are generated by a vapor-phase method such as a thermal plasma method. However, if the core particles are very small, say, having an average size of 1 xcexcm or less, agglomeration is likely to occur making it difficult to give monodisperse particles and, hence, effectively coated core particles. To deal with this problem, the core particles are kept as large as 0.1-100 xcexcm in average size and coated with the superfine particles, with the result that one can produce only large coated particles. In addition, the coated particles do not have a completely surrounding film. It should also be noted that the disclosure is substantially limited to the case where the superfine coating particles are also made of metal if the core particles are made of metal; in other words, there is no teaching of coating fine metal particles with an oxide of a dissimilar metal to produce oxide coated, fine metal particles.
Unexamined Published Japanese Patent Application No. 54008/1995, supra, discloses alumina coated quasi-fine TiAl particles comprising TiAl quasi-fine core particles with an average size of 40 xcexcm that are coated with superfine alumina (Al2O3) particles. However, the core particles are not smaller than 1 xcexcm and the coating alumina is not an oxide of a dissimilar metal, but similar to the metal in the main component of the core particles.
As described above, the coated particles available to date comprise large core particles, use metal coatings if the core particles are made of metal, and apply inorganic coatings if the core particles are made of inorganic materials. These coated particles are useful in sinters and thermal sprayed parts of the types described above but are not suitable for use in artificial bones with which strength and biocompatibility are two major concerns, and electrode materials in fuel cells that require high strength and good adhesion to various inorganic materials. Therefore, it has been strongly desired to develop oxide coated fine metal particles comprising fine metal particles coated with an oxide of a dissimilar metal.
The present invention has been accomplished under these circumstances and has as an object providing novel oxide coated fine metal particles comprising fine core metal particles that are coated ruggedly, preferably over the entire surfaces, with an oxide that does not contain as a main component the metal element which is the main component of the fine core metal particles.
Another object of the invention is to provide a process for producing the novel oxide coated fine metal particles in a positive and easy way.
The first object of the invention can be attained by oxide coated fine metal particles comprising fine core metal particles that are covered with a coating layer comprising either an oxide, a complex oxide or an oxy-acid salt that do not contain as a main component a metal element which is the main component of a fine core metal particles, or a complex oxide or a complex salt of the oxide, the complex oxide or the oxy-acid salt and an oxide of the metal element.
Preferably, the core particles have an average size of 0.01-1 xcexcm and the coating layer has an average thickness of 1-10 nm.
In a preferred embodiment, the metal element which is the main component of the fine core metal particles is at least one member of the group consisting of Al, Ti, V, Cr, Fe, Co, Ni, Mn, Cu, Zn, Zr, Ru, Pd, Ag, In, Pt, Au and Sm, and wherein the oxide, the complex oxide or the oxy-acid salt with which the fine core metal particles are coated is at least one member of the group consisting of titanium oxide, zirconium oxide, calcium oxide, silicon oxide, aluminum oxide, silver oxide, iron oxide, magnesium oxide, manganese oxide, yttrium oxide, cerium oxide, samarium oxide, beryllium oxide, barium titanate, lead titanate, lithium aluminate, yttrium vanadate, calcium phosphate, calcium zirconate, lead titanate zirconate, iron titanium oxide, cobalt titanium oxide and barium stannate.
The second object of the invention can be attained by a process for producing oxide coated fine metal particles, comprising the steps of: mixing a metal powder material with an oxide powder material of an oxide, a complex oxide or an oxy-acid salt that do not contain as a main component a metal element which is the main component of the metal powder material to obtain a powder material mixture; supplying the powder material mixture into a thermal plasma to make a vapor-phase mixture; and then quenching the vapor-phase mixture to form oxide coated fine metal particles comprising fine core metal particles that are finer than the metal powder material and which are covered with a coating layer comprising either the oxide, the complex oxide or the oxy-acid salt, or a complex oxide or a complex salt of the oxide, the complex oxide or the oxy-acid salt and an oxide of the metal element.
Preferably, the core particles have an average size of 0.01-1 xcexcm and the coating layer has an average thickness of 1-10 nm.
In a preferred embodiment, the metal element which is the main component of the fine core metal particles is at least one member of the group consisting of Al, Ti, V, Cr, Fe, Co, Ni, Mn, Cu, Zn, Zr, Ru, Pd, Ag, In, Pt, Au and Sm, and wherein the oxide, the complex oxide or the oxy-acid salt with which the fine core metal particles are coated is at least one member of the group consisting of titanium oxide, zirconium oxide, calcium oxide, silicon oxide, aluminum oxide, silver oxide, iron oxide, magnesium oxide, manganese oxide, yttrium oxide, cerium oxide, samarium oxide, beryllium oxide, barium titanate, lead titanate, lithium aluminate, yttrium vanadate, calcium phosphate, calcium zirconate, lead titanate zirconate, iron titanium oxide, cobalt titanium oxide and barium stannate.
Preferably, the metal powder material has an average particle size of 0.5-20 xcexcm and more preferably, all the particles in the feed are 20 xcexcm and smaller; the oxide powder material has preferably an average particle size of 0.1-1 xcexcm.
In a preferred embodiment, the metal powder material and the oxide powder material are mixed with a high-speed shear and impact mixer or a milling mixer. In another preferred embodiment, the powder material mixture of the metal powder material and the oxide powder material is an aggregate of composite particles having the individual particles in the metal powder material coated with the oxide powder material.
Preferably, the thermal plasma has a higher temperature than boiling points of the metal powder material and the oxide powder material.
In a preferred embodiment, the thermal plasma is in an atmosphere at 760 mmHg or below. In another preferred embodiment, the thermal plasma is in an atmosphere at 200-600 Torr.
Preferably, the vapor-phase mixture is quenched in an inert or reducing atmosphere; it is also preferred that the vapor-phase mixture is quenched in an atmosphere containing a rare gas either independently or in admixture with hydrogen.